In JavaScript, let
, var
, and const
are used to declare variables.
They have some similarities but also some important differences.
The main differences between let
, var
, and const
are as follows:
- Scope:
Variables declared withvar
have function scope or global scope, meaning they are accessible throughout the entire function or global environment. On the other hand, variables declared withlet
andconst
have block scope, which means they are only accessible within the nearest enclosing block (typically within curly braces {}). - Hoisting:
Variables declared withvar
are hoisted, which means that they are moved to the top of their scope during the compilation phase. This allows you to accessvar
variables before they are declared in the code. Variables declared withlet
andconst
, however, are not hoisted. They are only accessible after they are declared. - Reassignment:
Variables declared withvar
andlet
can be reassigned to a new value. For example, you can writevar x = 5; x = 10;
orlet y = 7; y = 15;
. On the other hand, variables declared withconst
are constants, which means their value cannot be changed once assigned. If you try to reassign aconst
variable, it will result in an error. - Temporal Dead Zone (TDZ):
Variables declared withlet
andconst
are subject to the concept of TDZ. This means that if you try to access them before they are declared, you will get a ReferenceError. This is different fromvar
, which can be accessed before its declaration (although it will have the valueundefined
). - Block redeclaration: Variables declared with
var
can be redeclared within the same scope without causing an error. This can lead to potential bugs and confusion. Variables declared withlet
andconst
, however, cannot be redeclared within the same block scope. Attempting to do so will result in a SyntaxError.
In general, it is recommended to use let
when you need to reassign a variable, and const
when you want to declare a constant that will not be reassigned.
The use of var
is less common nowadays, but it still has its use cases, especially when you need function or global scope variables.